Organic light emitting device having a transparent microcavity

ABSTRACT

An organic light emitting device having a microcavity is provided. The device may be transparent to the resonant wavelength of the microcavity, allowing for saturated emission at the wavelength or wavelengths of light transmitted by the microcavity.

FIELD OF THE INVENTION

The present invention relates to organic light emitting devices (OLEDs),and more specifically to OLEDs having a microcavity structure.

BACKGROUND

Opto-electronic devices that make use of organic materials are becomingincreasingly desirable for a number of reasons. Many of the materialsused to make such devices are relatively inexpensive, so organicopto-electronic devices have the potential for cost advantages overinorganic devices. In addition, the inherent properties of organicmaterials, such as their flexibility, may make them well suited forparticular applications such as fabrication on a flexible substrate.Examples of organic opto-electronic devices include organic lightemitting devices (OLEDs), organic phototransistors, organic photovoltaiccells, and organic photodetectors. For OLEDs, the organic materials mayhave performance advantages over conventional materials. For example,the wavelength at which an organic emissive layer emits light maygenerally be readily tuned with appropriate dopants.

As used herein, the term “organic” includes polymeric materials as wellas small molecule organic materials that may be used to fabricateorganic opto-electronic devices. “Small molecule” refers to any organicmaterial that is not a polymer, and “small molecules” may actually bequite large. Small molecules may include repeat units in somecircumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be a fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule,” and it isbelieved that all dendrimers currently used in the field of OLEDs aresmall molecules. In general, a small molecule has a well-definedchemical formula with a single molecular weight, whereas a polymer has achemical formula and a molecular weight that may vary from molecule tomolecule. As used herein, “organic” includes metal complexes ofhydrocarbyl and heteroatom-substituted hydrocarbyl ligands.

OLEDs make use of thin organic films that emit light when voltage isapplied across the device. OLEDs are becoming an increasinglyinteresting technology for use in applications such as flat paneldisplays, illumination, and backlighting. Several OLED materials andconfigurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and5,707,745, which are incorporated herein by reference in their entirety.

OLED devices are generally (but not always) intended to emit lightthrough at least one of the electrodes, and one or more transparentelectrodes may be useful in an organic opto-electronic devices. Forexample, a transparent electrode material, such as indium tin oxide(ITO), may be used as the bottom electrode. A transparent top electrode,such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745, which areincorporated by reference in their entireties, may also be used. For adevice intended to emit light only through the bottom electrode, the topelectrode does not need to be transparent, and may be comprised of athick and reflective metal layer having a high electrical conductivity.Similarly, for a device intended to emit light only through the topelectrode, the bottom electrode may be opaque and/or reflective. Wherean electrode does not need to be transparent, using a thicker layer mayprovide better conductivity, and using a reflective electrode mayincrease the amount of light emitted through the other electrode, byreflecting light back towards the transparent electrode. Fullytransparent devices may also be fabricated, where both electrodes aretransparent. Side emitting OLEDs may also be fabricated, and one or bothelectrodes may be opaque or reflective in such devices.

The light emitted by OLEDs may be characterized using known standards.For example, CIE (“Commission Internationale d'Eclairage”) is arecognized two-coordinate measure of the color of light. Ideal whitelight has a CIE of (0.33, 0.33). For white light sources, CRI (“ColorRendering Index”) is a recognized measure of the color shift that anobject undergoes when illuminated by the light source as compared withthe color of the same object when illuminated by a reference sourcecomparable to daylight. CRI values range from 0 to 100, with 100representing no color shift. Bright sunlight may have a CRI of 100.Fluorescent light bulbs have a CRI of 60-99, mercury lamps near 50, andhigh-pressure sodium lamps can have a CRI of about 20. Lamps used forhome or office lighting, for example, generally must meet very strictCIE and CRI requirements, whereas lamps used for street lighting, forexample, may be subject to more lenient CIE and CRI requirements.

As used herein, “top” means furthest away from the substrate, while“bottom” means closest to the substrate. For example, for a devicehaving two electrodes, the bottom electrode is the electrode closest tothe substrate, and is generally the first electrode fabricated. Thebottom electrode has two surfaces, a bottom surface closest to thesubstrate, and a top surface further away from the substrate. Where afirst layer is described as “disposed over” a second layer, the firstlayer is disposed further away from substrate. There may be other layersbetween the first and second layer, unless it is specified that thefirst layer is “in physical contact with” the second layer. For example,a cathode may be described as “disposed over” an anode, even thoughthere are various organic layers in between.

As used herein, “solution processible” means capable of being dissolved,dispersed, or transported in and/or deposited from a liquid medium,either in solution or suspension form.

As used herein, and as would be generally understood by one skilled inthe art, a first “Highest Occupied Molecular Orbital” (HOMO) or “LowestUnoccupied Molecular Orbital” (LUMO) energy level is “greater than” or“higher than” a second HOMO or LUMO energy level if the first energylevel is closer to the vacuum energy level. Since ionization potentials(IP) are measured as a negative energy relative to a vacuum level, ahigher HOMO energy level corresponds to an IP having a smaller absolutevalue (an IP that is less negative). Similarly, a higher LUMO energylevel corresponds to an electron affinity (EA) having a smaller absolutevalue (an EA that is less negative). On a conventional energy leveldiagram, with the vacuum level at the top, the LUMO energy level of amaterial is higher than the HOMO energy level of the same material. A“higher” HOMO or LUMO energy level appears closer to the top of such adiagram than a “lower” HOMO or LUMO energy level.

SUMMARY OF THE INVENTION

The present invention provides transparent microcavities. By adjustingthe reflectivity and spacing of the layers that define a microcavity, amicrocavity that is substantially transparent to light in the region ofthe resonant wavelength of the microcavity may be created. A transparentmicrocavity may be used in conjunction with or as part of an organiclight emitting device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an organic light emitting device having separate electrontransport, hole transport, and emissive layers, as well as other layers.

FIG. 2 shows an inverted organic light emitting device that does nothave a separate electron transport layer.

FIG. 3 shows the transmission spectra of two adjacent reflective layerswith no separation between the layers.

FIG. 4 shows the transmission spectra of two adjacent reflective layerswith a separation between the layers comparable to the wavelength ofvisible light.

FIG. 5 shows an OLED including a microcavity according to the invention.

FIG. 6 shows an OLED including a microcavity according to the invention.

FIG. 7 shows emission spectra for the transparent OLED shown in FIG. 6.

DETAILED DESCRIPTION

Generally, an OLED comprises at least one organic layer disposed betweenand electrically connected to an anode and a cathode. When a current isapplied, the anode injects holes and the cathode injects electrons intothe organic layer(s). The injected holes and electrons each migratetoward the oppositely charged electrode. When an electron and holelocalize on the same molecule, an “exciton,” which is a localizedelectron-hole pair having an excited energy state, is formed. Light isemitted when the exciton relaxes via a photoemissive mechanism. In somecases, the exciton may be localized on an excimer or an exciplex.Non-radiative mechanisms, such as thermal relaxation, may also occur,but are generally considered undesirable.

The initial OLEDs used emissive molecules that emitted light from theirsinglet states (“fluorescence”) as disclosed, for example, in U.S. Pat.No. 4,769,292, which is incorporated by reference in its entirety.Fluorescent emission generally occurs in a time frame of less than 10nanoseconds.

More recently, OLEDs having emissive materials that emit light fromtriplet states (“phosphorescence”) have been demonstrated. Baldo et al.,“Highly Efficient Phosphorescent Emission from OrganicElectroluminescent Devices,” Nature, vol. 395, 151-154, 1998;(“Baldo-I”) and Baldo et al., “Very high-efficiency green organiclight-emitting devices based on electrophosphorescence,” Appl. Phys.Lett., vol. 75, No. 1, 4-6 (1999) (“Baldo-II”), which are incorporatedby reference in their entireties. Phosphorescence may be referred to asa “forbidden” transition because the transition requires a change inspin states, and quantum mechanics indicates that such a transition isnot favored. As a result, phosphorescence generally occurs in a timeframe exceeding at least 10 nanoseconds, and typically greater than 100nanoseconds. If the natural radiative lifetime of phosphorescence is toolong, triplets may decay by a non-radiative mechanism, such that nolight is emitted. Organic phosphorescence is also often observed inmolecules containing heteroatoms with unshared pairs of electrons atvery low temperatures. 2,2′-bipyridine is such a molecule. Non-radiativedecay mechanisms are typically temperature dependent, such that anorganic material that exhibits phosphorescence at liquid nitrogentemperatures typically does not exhibit phosphorescence at roomtemperature. But, as demonstrated by Baldo, this problem may beaddressed by selecting phosphorescent compounds that do phosphoresce atroom temperature. Representative emissive layers include doped orun-doped phosphorescent organometallic materials such as disclosed inU.S. Pat. Nos. 6,303,238 and 6,310,360; U.S. Patent ApplicationPublication Nos. 2002-0034656; 2002-0182441; 2003-0072964; andWO-02/074015.

Generally, the excitons in an OLED are believed to be created in a ratioof about 3:1, i.e., approximately 75% triplets and 25% singlets. See,Adachi et al., “Nearly 100% Internal Phosphorescent Efficiency In AnOrganic Light Emitting Device,” J. Appl. Phys., 90, 5048 (2001), whichis incorporated by reference in its entirety. In many cases, singletexcitons may readily transfer their energy to triplet excited states via“intersystem crossing,” whereas triplet excitons may not readilytransfer their energy to singlet excited states. As a result, 100%internal quantum efficiency is theoretically possible withphosphorescent OLEDs. In a fluorescent device, the energy of tripletexcitons is generally lost to radiationless decay processes that heat-upthe device, resulting in much lower internal quantum efficiencies. OLEDsutilizing phosphorescent materials that emit from triplet excited statesare disclosed, for example, in U.S. Pat. No. 6,303,238, which isincorporated by reference in its entirety.

Phosphorescence may be preceded by a transition from a triplet excitedstate to an intermediate non-triplet state from which the emissive decayoccurs. For example, organic molecules coordinated to lanthanideelements often phosphoresce from excited states localized on thelanthanide metal. However, such materials do not phosphoresce directlyfrom a triplet excited state but instead emit from an atomic excitedstate centered on the lanthanide metal ion. The europium diketonatecomplexes illustrate one group of these types of species.

Phosphorescence from triplets can be enhanced over fluorescence byconfining, preferably through bonding, the organic molecule in closeproximity to an atom of high atomic number. This phenomenon, called theheavy atom effect, is created by a mechanism known as spin-orbitcoupling. Such a phosphorescent transition may be observed from anexcited metal-to-ligand charge transfer (MLCT) state of anorganometallic molecule such as tris(2-phenylpyridine)iridium(III).

As used herein, the term “triplet energy” refers to an energycorresponding to the highest energy feature discernable in thephosphorescence spectrum of a given material. The highest energy featureis not necessarily the peak having the greatest intensity in thephosphorescence spectrum, and could, for example, be a local maximum ofa clear shoulder on the high energy side of such a peak.

The term “organometallic” as used herein is as generally understood byone of ordinary skill in the art and as given, for example, in“Inorganic Chemistry” (2nd Edition) by Gary L. Miessler and Donald A.Tarr, Prentice Hall (1998). Thus, the term organometallic refers tocompounds which have an organic group bonded to a metal through acarbon-metal bond. This class does not include per se coordinationcompounds, which are substances having only donor bonds fromheteroatoms, such as metal complexes of amines, halides, pseudohalides(CN, etc.), and the like. In practice organometallic compounds generallycomprise, in addition to one or more carbon-metal bonds to an organicspecies, one or more donor bonds from a heteroatom. The carbon-metalbond to an organic species refers to a direct bond between a metal and acarbon atom of an organic group, such as phenyl, alkyl, alkenyl, etc.,but does not refer to a metal bond to an “inorganic carbon,” such as thecarbon of CN or CO.

FIG. 1 shows an organic light emitting device 100. The figures are notnecessarily drawn to scale. Device 100 may include a substrate 110, ananode 115, a hole injection layer 120, a hole transport layer 125, anelectron blocking layer 130, an emissive layer 135, a hole blockinglayer 140, an electron transport layer 145, an electron injection layer150, a protective layer 155, and a cathode 160. Cathode 160 is acompound cathode having a first conductive layer 162 and a secondconductive layer 164. Device 100 may be fabricated by depositing thelayers described, in order.

Substrate 110 may be any suitable substrate that provides desiredstructural properties. Substrate 110 may be flexible or rigid. Substrate110 may be transparent, translucent or opaque. Plastic and glass areexamples of preferred rigid substrate materials. Plastic and metal foilsare examples of preferred flexible substrate materials. Substrate 110may be a semiconductor material in order to facilitate the fabricationof circuitry. For example, substrate 110 may be a silicon wafer uponwhich circuits are fabricated, capable of controlling OLEDs subsequentlydeposited on the substrate. Other substrates may be used. The materialand thickness of substrate 110 may be chosen to obtain desiredstructural and optical properties.

Anode 115 may be any suitable anode that is sufficiently conductive totransport holes to the organic layers. The material of anode 115preferably has a work function higher than about 4 eV (a “high workfunction material”). Preferred anode materials include conductive metaloxides, such as indium tin oxide (ITO) and indium zinc oxide (IZO),aluminum zinc oxide (AlZnO), and metals. Anode 115 (and substrate 110)may be sufficiently transparent to create a bottom-emitting device. Apreferred transparent substrate and anode combination is commerciallyavailable ITO (anode) deposited on glass or plastic (substrate). Aflexible and transparent substrate-anode combination is disclosed inU.S. Pat. Nos. 5,844,363 and 6,602,540 B2, which are incorporated byreference in their entireties. Anode 115 may be opaque and/orreflective. A reflective anode 115 may be preferred for sometop-emitting devices, to increase the amount of light emitted from thetop of the device. The material and thickness of anode 115 may be chosento obtain desired conductive and optical properties. Where anode 115 istransparent, there may be a range of thickness for a particular materialthat is thick enough to provide the desired conductivity, yet thinenough to provide the desired degree of transparency. Other anodematerials and structures may be used.

Hole transport layer 125 may include a material capable of transportingholes. Hole transport layer 130 may be intrinsic (undoped), or doped.Doping may be used to enhance conductivity. α-NPD and TPD are examplesof intrinsic hole transport layers. An example of a p-doped holetransport layer is m-MTDATA doped with F₄-TCNQ at a molar ratio of 50:1,as disclosed in United States Patent Application Publication No.2003-0230980 to Forrest et al., which is incorporated by reference inits entirety. Other hole transport layers may be used.

Emissive layer 135 may include an organic material capable of emittinglight when a current is passed between anode 115 and cathode 160.Preferably, emissive layer 135 contains a phosphorescent emissivematerial, although fluorescent emissive materials may also be used.Phosphorescent materials are preferred because of the higher luminescentefficiencies associated with such materials. Emissive layer 135 may alsocomprise a host material capable of transporting electrons and/or holes,doped with an emissive material that may trap electrons, holes, and/orexcitons, such that excitons relax from the emissive material via aphotoemissive mechanism. Emissive layer 135 may comprise a singlematerial that combines transport and emissive properties. Whether theemissive material is a dopant or a major constituent, emissive layer 135may comprise other materials, such as dopants that tune the emission ofthe emissive material. Emissive layer 135 may include a plurality ofemissive materials capable of, in combination, emitting a desiredspectrum of light. Examples of phosphorescent emissive materials includeIr(ppy)₃. Examples of fluorescent emissive materials include DCM andDMQA. Examples of host materials include Alq₃, CBP and mCP. Examples ofemissive and host materials are disclosed in U.S. Pat. No. 6,303,238 toThompson et al., which is incorporated by reference in its entirety.Emissive material may be included in emissive layer 135 in a number ofways. For example, an emissive small molecule may be incorporated into apolymer. This may be accomplished by several ways: by doping the smallmolecule into the polymer either as a separate and distinct molecularspecies; or by incorporating the small molecule into the backbone of thepolymer, so as to form a co-polymer; or by bonding the small molecule asa pendant group on the polymer. Other emissive layer materials andstructures may be used. For example, a small molecule emissive materialmay be present as the core of a dendrimer.

Many useful emissive materials include one or more ligands bound to ametal center. A ligand may be referred to as “photoactive” if itcontributes directly to the photoactive properties of an organometallicemissive material. A “photoactive” ligand may provide, in conjunctionwith a metal, the energy levels from which and to which an electronmoves when a photon is emitted. Other ligands may be referred to as“ancillary.” Ancillary ligands may modify the photoactive properties ofthe molecule, for example by shifting the energy levels of a photoactiveligand, but ancillary ligands do not directly provide the energy levelsinvolved in light emission. A ligand that is photoactive in one moleculemay be ancillary in another. These definitions of photoactive andancillary are intended as non-limiting theories.

Electron transport layer 145 may include a material capable oftransporting electrons. Electron transport layer 145 may be intrinsic(undoped), or doped. Doping may be used to enhance conductivity. Alq₃ isan example of an intrinsic electron transport layer. An example of ann-doped electron transport layer is BPhen doped with Li at a molar ratioof 1:1, as disclosed in United States Patent Application Publication No.2003-0230980 to Forrest et al., which is incorporated by reference inits entirety. Other electron transport layers may be used.

The charge carrying component of the electron transport layer may beselected such that electrons can be efficiently injected from thecathode into the LUMO (Lowest Unoccupied Molecular Orbital) energy levelof the electron transport layer. The “charge carrying component” is thematerial responsible for the LUMO energy level that actually transportselectrons. This component may be the base material, or it may be adopant. The LUMO energy level of an organic material may be generallycharacterized by the electron affinity of that material and the relativeelectron injection efficiency of a cathode may be generallycharacterized in terms of the work function of the cathode material.This means that the preferred properties of an electron transport layerand the adjacent cathode may be specified in terms of the electronaffinity of the charge carrying component of the ETL and the workfunction of the cathode material. In particular, so as to achieve highelectron injection efficiency, the work function of the cathode materialis preferably not greater than the electron affinity of the chargecarrying component of the electron transport layer by more than about0.75 eV, more preferably, by not more than about 0.5 eV. Similarconsiderations apply to any layer into which electrons are beinginjected.

Cathode 160 may be any suitable material or combination of materialsknown to the art, such that cathode 160 is capable of conductingelectrons and injecting them into the organic layers of device 100.Cathode 160 may be transparent or opaque, and may be reflective. Metalsand metal oxides are examples of suitable cathode materials. Cathode 160may be a single layer, or may have a compound structure. FIG. 1 shows acompound cathode 160 having a thin metal layer 162 and a thickerconductive metal oxide layer 164. In a compound cathode, preferredmaterials for the thicker layer 164 include ITO, IZO, and othermaterials known to the art. U.S. Pat. Nos. 5,703,436, 5,707,745,6,548,956 B2 and 6,576,134 B2, which are incorporated by reference intheir entireties, disclose examples of cathodes including compoundcathodes having a thin layer of metal such as Mg:Ag with an overlyingtransparent, electrically-conductive, sputter-deposited ITO layer. Thepart of cathode 160 that is in contact with the underlying organiclayer, whether it is a single layer cathode 160, the thin metal layer162 of a compound cathode, or some other part, is preferably made of amaterial having a work function lower than about 4 eV (a “low workfunction material”). Other cathode materials and structures may be used.

Blocking layers may be used to reduce the number of charge carriers(electrons or holes) and/or excitons that leave the emissive layer. Anelectron blocking layer 130 may be disposed between emissive layer 135and the hole transport layer 125, to block electrons from leavingemissive layer 135 in the direction of hole transport layer 125.Similarly, a hole blocking layer 140 may be disposed between emissivelayer 135 and electron transport layer 145, to block holes from leavingemissive layer 135 in the direction of electron transport layer 145.Blocking layers may also be used to block excitons from diffusing out ofthe emissive layer. The theory and use of blocking layers is describedin more detail in U.S. Pat. No. 6,097,147 and United States PatentApplication Publication No. 2003-0230980 to Forrest et al., which areincorporated by reference in their entireties.

As used herein, and as would be understood by one skilled in the art,the term “blocking layer” means that the layer provides a barrier thatsignificantly inhibits transport of charge carriers and/or excitonsthrough the device, without suggesting that the layer necessarilycompletely blocks the charge carriers and/or excitons. The presence ofsuch a blocking layer in a device may result in substantially higherefficiencies as compared to a similar device lacking a blocking layer.Also, a blocking layer may be used to confine emission to a desiredregion of an OLED.

Generally, injection layers are comprised of a material that may improvethe injection of charge carriers from one layer, such as an electrode oran organic layer, into an adjacent organic layer. Injection layers mayalso perform a charge transport function. In device 100, hole injectionlayer 120 may be any layer that improves the injection of holes fromanode 115 into hole transport layer 125. CuPc is an example of amaterial that may be used as a hole injection layer from an ITO anode115, and other anodes. In device 100, electron injection layer 150 maybe any layer that improves the injection of electrons into electrontransport layer 145. LiF/Al is an example of a material that may be usedas an electron injection layer into an electron transport layer from anadjacent layer. Other materials or combinations of materials may be usedfor injection layers. Depending upon the configuration of a particulardevice, injection layers may be disposed at locations different thanthose shown in device 100. More examples of injection layers areprovided in U.S. patent application Ser. No. 09/931,948 to Lu et al.,which is incorporated by reference in its entirety. A hole injectionlayer may comprise a solution deposited material, such as a spin-coatedpolymer, e.g., PEDOT:PSS, or it may be a vapor deposited small moleculematerial, e.g., CuPc or MTDATA.

A hole injection layer (HIL) may planarize or wet the anode surface soas to provide efficient hole injection from the anode into the holeinjecting material. A hole injection layer may also have a chargecarrying component having HOMO (Highest Occupied Molecular Orbital)energy levels that favorably match up, as defined by theirherein-described relative ionization potential (IP) energies, with theadjacent anode layer on one side of the HIL and the hole transportinglayer on the opposite side of the HIL. The “charge carrying component”is the material responsible for the HOMO energy level that actuallytransports holes. This component may be the base material of the HIL, orit may be a dopant. Using a doped HIL allows the dopant to be selectedfor its electrical properties, and the host to be selected formorphological properties such as wetting, flexibility, toughness, etc.Preferred properties for the HIL material are such that holes can beefficiently injected from the anode into the HIL material. Inparticular, the charge carrying component of the HIL preferably has anIP not more than about 0.7 eV greater that the IP of the anode material.More preferably, the charge carrying component has an IP not more thanabout 0.5 eV greater than the anode material. Similar considerationsapply to any layer into which holes are being injected. HIL materialsare further distinguished from conventional hole transporting materialsthat are typically used in the hole transporting layer of an OLED inthat such HIL materials may have a hole conductivity that issubstantially less than the hole conductivity of conventional holetransporting materials. The thickness of the HIL of the presentinvention may be thick enough to help planarize or wet the surface ofthe anode layer. For example, an HIL thickness of as little as 10 nm maybe acceptable for a very smooth anode surface. However, since anodesurfaces tend to be very rough, a thickness for the HIL of up to 50 nmmay be desired in some cases.

A protective layer may be used to protect underlying layers duringsubsequent fabrication processes. For example, the processes used tofabricate metal or metal oxide top electrodes may damage organic layers,and a protective layer may be used to reduce or eliminate such damage.In device 100, protective layer 155 may reduce damage to underlyingorganic layers during the fabrication of cathode 160. Preferably, aprotective layer has a high carrier mobility for the type of carrierthat it transports (electrons in device 100), such that it does notsignificantly increase the operating voltage of device 100. CuPc, BCP,and various metal phthalocyanines are examples of materials that may beused in protective layers. Other materials or combinations of materialsmay be used. The thickness of protective layer 155 is preferably thickenough that there is little or no damage to underlying layers due tofabrication processes that occur after organic protective layer 160 isdeposited, yet not so thick as to significantly increase the operatingvoltage of device 100. Protective layer 155 may be doped to increase itsconductivity. For example, a CuPc or BCP protective layer 160 may bedoped with Li. A more detailed description of protective layers may befound in U.S. patent application Ser. No. 09/931,948 to Lu et al., whichis incorporated by reference in its entirety.

FIG. 2 shows an inverted OLED 200. The device includes a substrate 210,an cathode 215, an emissive layer 220, a hole transport layer 225, andan anode 230. Device 200 may be fabricated by depositing the layersdescribed, in order. Because the most common OLED configuration has acathode disposed over the anode, and device 200 has cathode 215 disposedunder anode 230, device 200 may be referred to as an “inverted” OLED.Materials similar to those described with respect to device 100 may beused in the corresponding layers of device 200. FIG. 2 provides oneexample of how some layers may be omitted from the structure of device100.

The simple layered structure illustrated in FIGS. 1 and 2 is provided byway of non-limiting example, and it is understood that embodiments ofthe invention may be used in connection with a wide variety of otherstructures. The specific materials and structures described areexemplary in nature, and other materials and structures may be used.Functional OLEDs may be achieved by combining the various layersdescribed in different ways, or layers may be omitted entirely, based ondesign, performance, and cost factors. Other layers not specificallydescribed may also be included. Materials other than those specificallydescribed may be used. Although many of the examples provided hereindescribe various layers as comprising a single material, it isunderstood that combinations of materials, such as a mixture of host anddopant, or more generally a mixture, may be used. Also, the layers mayhave various sublayers. The names given to the various layers herein arenot intended to be strictly limiting. For example, in device 200, holetransport layer 225 transports holes and injects holes into emissivelayer 220, and may be described as a hole transport layer or a holeinjection layer. In one embodiment, an OLED may be described as havingan “organic layer” disposed between a cathode and an anode. This organiclayer may comprise a single layer, or may further comprise multiplelayers of different organic materials as described, for example, withrespect to FIGS. 1 and 2.

Structures and materials not specifically described may also be used,such as OLEDs comprised of polymeric materials (PLEDs) such as disclosedin U.S. Pat. No. 5,247,190, Friend et al., which is incorporated byreference in its entirety. By way of further example, OLEDs having asingle organic layer may be used. OLEDs may be stacked, for example asdescribed in U.S. Pat. No. 5,707,745 to Forrest et al, which isincorporated by reference in its entirety. The OLED structure maydeviate from the simple layered structure illustrated in FIGS. 1 and 2.For example, the substrate may include an angled reflective surface toimprove out-coupling, such as a mesa structure as described in U.S. Pat.No. 6,091,195 to Forrest et al., and/or a pit structure as described inU.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated byreference in their entireties.

Unless otherwise specified, any of the layers of the various embodimentsmay be deposited by any suitable method. For the organic layers,preferred methods include thermal evaporation, ink-jet, such asdescribed in U.S. Pat. Nos. 6,013,982 and 6,087,196, which areincorporated by reference in their entireties, organic vapor phasedeposition (OVPD), such as described in U.S. Pat. No. 6,337,102 toForrest et al., which is incorporated by reference in its entirety, anddeposition by organic vapor jet printing (OVJP), such as described inU.S. patent application Ser. No. 10/233,470, which is incorporated byreference in its entirety. Other suitable deposition methods includespin coating and other solution based processes. Solution basedprocesses are preferably carried out in nitrogen or an inert atmosphere.For the other layers, preferred methods include thermal evaporation.Preferred patterning methods include deposition through a mask, coldwelding such as described in U.S. Pat. Nos. 6,294,398 and 6,468,819,which are incorporated by reference in their entireties, and patterningassociated with some of the deposition methods such as ink-jet and OVJP.Other methods may also be used. The materials to be deposited may bemodified to make them compatible with a particular deposition method.For example, substituents such as alkyl and aryl groups, branched orunbranched, and preferably containing at least 3 carbons, may be used insmall molecules to enhance their ability to undergo solution processing.Substituents having 20 carbons or more may be used, and 3-20 carbons isa preferred range. Materials with asymmetric structures may have bettersolution processibility than those having symmetric structures, becauseasymmetric materials may have a lower tendency to recrystallize.Dendrimer substituents may be used to enhance the ability of smallmolecules to undergo solution processing.

The molecules disclosed herein may be substituted in a number ofdifferent ways without departing from the scope of the invention. Forexample, substituents may be added to a compound having three bidentateligands, such that after the substituents are added, one or more of thebidentate ligands are linked together to form, for example, atetradentate or hexadentate ligand. Other such linkages may be formed.It is believed that this type of linking may increase stability relativeto a similar compound without linking, due to what is generallyunderstood in the art as a “chelating effect.”

Devices fabricated in accordance with embodiments of the invention maybe incorporated into a wide variety of consumer products, including flatpanel displays, computer monitors, televisions, billboards, lights forinterior or exterior illumination and/or signaling, heads up displays,fully transparent displays, flexible displays, laser printers,telephones, cell phones, personal digital assistants (PDAs), laptopcomputers, digital cameras, camcorders, viewfinders, micro-displays,vehicles, a large area wall, theater or stadium screen, or a sign.Various control mechanisms may be used to control devices fabricated inaccordance with the present invention, including passive matrix andactive matrix. Many of the devices are intended for use in a temperaturerange comfortable to humans, such as 18 degrees C. to 30 degrees C., andmore preferably at room temperature (20-25 degrees C.).

The materials and structures described herein may have applications indevices other than OLEDs. For example, other optoelectronic devices suchas organic solar cells and organic photodetectors may employ thematerials and structures. More generally, organic devices, such asorganic transistors, may employ the materials and structures.

OLEDs may be constructed such that electrodes or other reflective orsemi-reflective layers of the device define a microcavity. A microcavitymay be formed when the optical distance between two reflective orsemi-reflective layers has a magnitude that is comparable to thewavelength of visible light. The transmission of the microcavity maythen exceed the transmission of the individual reflective orsemi-reflective layers at a resonant wavelength of the microcavity. Asused herein the two layers may be referred to as “defining” amicrocavity when the layers meet these criteria.

The resonant peaks in the transmission spectrum of a microcavity may becontrolled by adjusting the reflectivity of the layers defining themicrocavity and the separation between the layers. In general,microcavities may be constructed that have one transparent orsemitransparent reflective layer and one opaque reflective layer. Theemission in the forward direction (i.e., through the transparent orsemitransparent reflective layer) may be calculated as:

$\begin{matrix}{{{E_{c}(\lambda)}}^{2} = {\frac{\left( {1 - R_{d}} \right)\left\lbrack {1 + R_{m} + {2\sqrt{R_{m}}{\cos\left( \frac{4\;\pi\; x}{\lambda} \right)}}} \right\rbrack}{1 + {\sqrt{R_{m}R_{d}}{\cos\left( \frac{4\;\pi\; L}{\lambda} \right)}}}{{E_{n}(\lambda)}}^{2}}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$where λ is the emission wavelength, x is the effective distance of theemissive layer from the opaque layer, R_(m) and R_(d) are thereflectivities of the opaque mirror and the transparent mirror,respectively, L is the total optical length of the microcavity, andE_(n)(λ) is an original (free-space) spectrum. The optical length of themicrocavity, L, may be given by:

$\begin{matrix}{L = {{\frac{\lambda}{2}\left( \frac{n_{eff}}{\Delta\; n} \right)} + {\sum\limits_{i}{n_{i}d_{i}}} + {{\frac{\varphi_{m}}{4\;\pi}\lambda}}}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$where n_(eff) and Δn are the effective refractive index and the indexdifference between the reflective layers, n_(i) and d_(i) are therefractive index and the thickness of the organic layers and thetransparent layer, and φ_(m) is the phase change at the opaque mirror.

A microcavity may be characterized by its resonant wavelength. Amicrocavity generally emits saturated light in one or more relativelynarrow regions of the visible spectrum, and may emit less or no light inother regions. The wavelengths corresponding to a peak or peaks in theemission spectrum may be referred to as the resonant wavelength orwavelengths of the microcavity.

A microcavity also may be characterized by its finesse. The finesse F ofa microcavity is defined as the ratio of the separation between resonantpeaks in the transmission spectrum of the microcavity, Δυ, to thefull-width at half-maximum (FWHM) of the resonant frequency peak of thespectrum, Δυ_(1/2): F=Δυ/Δυ_(1/2). In general, a microcavity will have afinesse greater than about 1.5. It may be preferred for microcavitiesused in conjunction with the present invention to have a finesse of atleast 1.5-5; a finesse of 5 corresponds to a spectrum having, forexample, a FWHM of 60 nm and resonant peak separation of 300 nm.

In some cases, “standard” OLEDs have been described as having“microcavity effects.” These effects are usually undesirable. Forexample, a device may quench, prevent, or otherwise diminish emission atsome wavelengths, and/or increase emission at certain wavelengths atwhich the device emits. While these and similar effects may be referredto as “microcavity effects,” these devices are not considered to containor define a “microcavity.” As used herein, a “standard” OLED is onewhich does not have layers defining a microcavity; two layers, such astwo electrodes disposed on either side of an organic layer, may be saidto define a standard OLED when the reflectivities and spacing of thelayers does not result in the formation of a microcavity. A standardOLED generally has a finesse of less than about 1.5. Such a device may,however, exhibit “microcavity effects” as discussed above. The organiclayer disposed between the layers that define a standard OLED maycomprise multiple layers or sublayers.

According to the invention, the transmission, reflectance, andseparation of two layers which define a microcavity may be adjusted tocreate a transparent microcavity. As used herein, a microcavity may bereferred to as a “transparent microcavity” if it transmits light in theregion of its resonant wavelength. A transparent microcavity maytransmit less light or fail to transmit light at all in a region of thevisible spectrum not corresponding to the resonant wavelength; hence“transparent” is not intended to signify complete transparency over thevisible spectrum. Transparent microcavities according to the presentinvention may be used to fabricate, for example, a material thattransmits a desired color or range of light. When such devices areactivated, the material emits light of the same color that it transmits.For example, a window may be fabricated that transmits green light; whenan appropriate voltage is applied to OLEDs in the window, the windowthen emits green light. Other colors may be used, and devices accordingto the invention may be suited for a wide variety of uses as previouslydescribed.

It may be preferred for each layer defining a transparent microcavity tohave a transmission over the visible spectrum of at most 45% and areflectance over the visible spectrum of at least 55%. The “narrowness”of the emission of a microcavity (i.e., the full width at half-maximumof a peak in the emission spectrum) may be adjusted by adjusting thereflectance of the layers defining the microcavity. The transmission andreflectance of the layers defining the microcavity may be adjusted, forexample, by adjusting the thickness of the layers or the amount ofreflective material in the layers.

For example, two 40 nm thick Ag layers have the transmission spectrumshown in FIG. 3 when the distance between the two layers is zero. Amicrocavity formed by inserting a 120 nm spacer between the layers hasthe transmission spectrum shown in FIG. 4. The transmission of thelayers alone is below 3% over the visible range, but the transmission ofthe microcavity at the resonant wavelength peaks at 30%. As shown inFIGS. 3-4, although the individual layers that define the transparentmicrocavity may not be transparent across the visual spectrum whenconsidered apart from the microcavity, the microcavity may betransparent over some wavelengths of visible light in the region of theresonant wavelength or wavelengths. In the example shown in FIGS. 3-4,the microcavity is transparent in the region of about 540 to about 620nm. It may be preferred for the microcavity to transmit less light atwavelengths other than the resonant wavelength, and in general it maytransmit 0-30% of light across a substantial portion of the visiblespectrum other than the resonant wavelength or a small range near theresonant wavelength.

It may be preferred for a microcavity to be transparent within someregion of the resonant wavelength of the microcavity. The full-width athalf-maximum (FWHM) of the resonant wavelength peak, or a convenientmultiple thereof, may be used as a range for which it may be preferredthat the microcavity transmit light. For example, the device describedwith respect to FIG. 3-4 has a transmission peak in the region of theresonant wavelength of the device, with the peak having a FWHM of about30 nm. For such a device, it may be preferred to adjust the microcavitysuch that the device is transparent to wavelengths of light within about30 nm of the resonant wavelength (i.e., up to 30 nm higher or lower thanthe resonant wavelength). It may be preferred for the device to transmit15-100% of light within this wavelength range, and more preferred thatit transmit 30-100% within the range. Such a range may be useful toallow transmission across a substantial portion of the resonantwavelength peak. It may also be preferred that the microcavity transmitless that 30% of light at some wavelengths outside this range. It may bepreferred for the microcavity to transmit 0-15% of light at somewavelengths outside the range. It may be preferred for the microcavityto transmit 0-30%, and more preferably 0-15%, of light at allwavelengths outside the range. For example, if a “green” transparentmicrocavity is desired, it may be preferred for the microcavity totransmit light primarily in the region of about 500 nm to 580 nm.Specifically, it may be preferred that it transmit 30-100% of light inthe 500-580 nm range, and less than 30% at other wavelengths, morepreferred that it transmit less than 15%, and more preferred that ittransmit less than 10% of light at other wavelengths. Other colors orranges may be used.

FIG. 5 shows a transparent microcavity according to the invention. Ananode 115 is deposited on a substrate 110 as previously described. Oneor more organic layers 170, 180 are disposed between the anode 115 and acathode 160. The organic layers 170, 180 may comprise various layers andsublayers, such as blocking layers and/or transport layers, aspreviously described. In some cases, one or more “spacers” may be used,where a “spacer” is a layer that does not substantially affect theemission characteristics or operation of the device. Spacers generallyare used to adjust the separation of the electrodes defining themicrocavity. In the device shown in FIG. 5, the anode 115 and thecathode 160 are arranged to define a microcavity 500. It may bepreferred to adjust the spacing and reflectivities of the anode andcathode such that the device transmits at least 30% of light at theresonant wavelength of the microcavity. It may also be preferred toadjust the anode and cathode such that the device transmits at least 15%of light within a range around the resonant wavelength, typically theFWHM of the resonant peak. Preferred materials for the cathode and/orthe electrode may include silver, aluminum, and compounds of silverand/or aluminum. Preferred materials for the organic layers may beselected for a desired emission spectrum based on emissioncharacteristics known in the art. The resonant wavelength of themicrocavity, and hence the location of peaks in the spectrum of lightemitted by the microcavity, may be adjusted by changing the reflectivityand relative positions of the anode 115 and the cathode 160.

It is understood that the various embodiments described herein are byway of example only, and are not intended to limit the scope of theinvention. For example, many of the materials and structures describedherein may be substituted with other materials and structures withoutdeviating from the spirit of the invention. It is understood thatvarious theories as to why the invention works are not intended to belimiting. For example, theories relating to charge transfer are notintended to be limiting.

Material Definitions:

As used herein, abbreviations refer to materials as follows:

-   CBP: 4,4′-N,N-dicarbazole-biphenyl-   m-MTDATA 4,4′,4″-tris(3-methylphenylphenlyamino)triphenylamine-   Alq₃: 8-tris-hydroxyquinoline aluminum-   Bphen: 4,7-diphenyl-1,10-phenanthroline-   n-BPhen: n-doped BPhen (doped with lithium)-   F₄-TCNQ: tetrafluoro-tetracyano-quinodimethane-   p-MTDATA: p-doped m-MTDATA (doped with F₄-TCNQ)-   Ir(ppy)₃: tris(2-phenylpyridine)-iridium-   Ir(ppz)₃: tris(1-phenylpyrazoloto,N,C(2′)iridium(III)-   BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline-   TAZ: 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole-   CuPc: copper phthalocyanine.-   ITO: indium tin oxide-   NPD: N,N′-diphenyl-N—N′-di(1-naphthyl)-benzidine-   TPD: N,N′-diphenyl-N—N′-di(3-toly)-benzidine-   BAlq:    aluminum(III)bis(2-methyl-8-hydroxyquinolinato)4-phenylphenolate-   mCP: 1,3-N,N-dicarbazole-benzene-   DCM: 4-(dicyanoethylene)-6-(4-dimethylaminostyryl-2-methyl)-4H-pyran-   DMQA: N,N′-dimethylquinacridone-   PEDOT:PSS: an aqueous dispersion of poly(3,4-ethylenedioxythiophene)    with polystyrenesulfonate (PSS)-   Ir(5-Phppy)₃: iridium(III)tris(5-phenyl-2-phenylpyridine)

EXPERIMENTAL

Specific representative embodiments of the invention will now bedescribed, including how such embodiments may be made. It is understoodthat the specific methods, materials, conditions, process parameters,apparatus and the like do not necessarily limit the scope of theinvention.

A device having the following structure was fabricated on a glasssubstrate: Ag (30 nm)/LGHIL (10 nm)/NPD (30 nm)/CBP:Ir(5-Phppy)₃ 9% (30nm)/Balq (10 nm)/Alq (32.5 nm)/LiF/Al (15 nm). A device schematic isshown in FIG. 6. The thickness of the various layers was chosen suchthat the Ag and LiF/Al layers formed a microcavity 600 having a resonantwavelength at about 540 nm. The electroluminescence of the device isshown in FIG. 7. Emission through the Ag anode 710 and through theLiF/AI cathode 720 are shown. The FWHM of each spectrum was 32 nm,resulting in light having a CIE of (0.32, 0.66) (saturated green). LGHILwas obtained from LG Chem Ltd., Seoul, South Korea. LGHIL has a purity(HPLC) of greater than 99.5%, a glass transition temperature of 250 C.,and a melting point greater than 400 C. Ir(5-Phppy)₃ has the followingstructure:

Further information regarding Ir(5-Phppy)₃ is given in U.S. ProvisionalApplication No. 60/814,314, filed Jun. 15, 2006, the disclosure of whichis incorporated by reference in its entirety.

While the present invention is described with respect to particularexamples and preferred embodiments, it is understood that the presentinvention is not limited to these examples and embodiments. The presentinvention as claimed therefore includes variations from the particularexamples and preferred embodiments described herein, as will be apparentto one of skill in the art.

1. A device comprising: an anode; a cathode; and an organic emissivelayer disposed between the anode and the cathode, wherein: the anode andthe cathode define a microcavity; one of the anode or the cathode, orboth, have a reflectance of at least 55% for light over the visiblespectrum and a transmission of at most 45% for light over the visiblespectrum; there are no electrodes between the anode and the cathode; thedevice transmits 30-100% of light at a resonant wavelength of themicrocavity; the device transmits 0-30% of light at least one wavelengthin the visible region not equal to a resonant wavelength of themicrocavity; and the spectrum of light emitted by the device has a peakwith a full-width at half-maximum of 60 nm or less.
 2. The device ofclaim 1, wherein the spectrum of light emitted by the device has a peakwith a full-width at half-maximum of 30 nm to 60 nm.
 3. The device ofclaim 1, wherein the microcavity has a finesse of 1.5 to
 5. 4. Thedevice of claim 1, wherein the device transmits 15-100% of light havinga wavelength within 30 nm of a resonant wavelength of the microcavity.5. The device of claim 1, further comprising an organic non-emissivelayer disposed between the anode and the single organic emissive layer.6. The device of claim 1, further comprising an organic non-emissivelayer disposed between the cathode and the single organic emissivelayer.
 7. The device of claim 1, wherein only a single organic emissivelayer is disposed between the anode and the cathode.
 8. The device ofclaim 1, wherein the cathode has a reflectance of at least 55% for lightover the visible spectrum and a transmission of at most 45% for lightover the visible spectrum.
 9. The device of claim 1, wherein the anodehas a reflectance of at least 55% for light over the visible spectrumand a transmission of at most 45% for light over the visible spectrum.